What workflow should we use to create a text object that requires characters from multiple .ttf fonts?
Not sure if it works for you, probably not a very solid or flexible long term solution,  but have you looked into Object Font?

Basically you specify a "family name" or a base naming convention for a bunch of objects, you then manually name any 3d object using that naming convention like fontobject_a fontobject_b fontobject_c fontobject_n etc.

You then enter this family name in the Properties Window > Object Data > Font > Object font as fontobject_ 

You can now specify any regular blender objects as a character replacement. You manually create objects for every character, can be a 3D mesh, a bezier curve or even another text object using the desired font file, and name it properly.

Turn on vertex duplication on the original font object under Properties Window > Object > Duplication > Verts and you can have as many different fonts as you like.

Have in mind that it is an admittedly cumbersome solution and will probably require post processing like making the duplicates real and hiding the underlying text object.